Stabilization of Ion Concentration Polarization Using a
Heterogeneous Nanoporous Junction
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Citation
Kim, Pilnam et al. “Stabilization of Ion Concentration Polarization
Using a Heterogeneous Nanoporous Junction.” Nano Letters
10.1 (2010): 16–23.
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http://dx.doi.org/10.1021/nl9023319
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American Chemical Society (ACS)
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Author's final manuscript
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Wed May 25 21:52:58 EDT 2016
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Author Manuscript
Nano Lett. Author manuscript; available in PMC 2011 January 1.
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Published in final edited form as:
Nano Lett. 2010 January ; 10(1): 16–23. doi:10.1021/nl9023319.
Stabilization of Ion Concentration Polarization Using a
Heterogeneous Nanoporous Junction
Pilnam Kim1,†, Sung Jae Kim2,†, Jongyoon Han2,3,*, and Kahp Y. Suh1,4,*
1School of Mechanical and Aerospace Engineering, Seoul National University, Seoul, 151-742,
Korea
2Department
of Electrical Engineering and Computer Science, Massachusetts Institute of
Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
3Department
of Biological Engineering, Massachusetts Institute of Technology, 77 Massachusetts
Avenue, Cambridge, MA 02139, USA
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4World
Class University Program on Multiscale Mechanical Design, Seoul National University,
Seoul, 151-742, Korea
Abstract
We demonstrate a recycled ion – flux through heterogeneous nanoporous junctions, which induce
stable ion concentration polarization (ICP) with an electric field. The nanoporous junctions are based
on integration of ionic hydrogels whose surfaces are negatively- and positively- charged for cationic
selectivity and anionic selectivity, respectively. It is shown that a ‘heterojunction’ structure with
cationic selective hydrogels (CSH) and anionic selective hydrogels (ASH) can be matched up in a
way to achieve continuous ion-flux operation for stable concentration gradient or ionic conductance.
Furthermore, the combined junctions can be used to accumulate ions on a specific region of the
device.
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Ion transport through nano-scale geometries has been extensively studied due to its potential
applications to biomolecule concentration1-4 and separation5, 6, fluid pumping 7 and
switching8, nanofluidic diode9, 10 and fuel cells11, 12. In particular, ion concentration
polarization (ICP) is important and useful in nanoscale ion transport, which can be generated
by perm-selective transport through nanochannels or nanopores, because the Debye layer
thickness (λD) is not negligible compared with the channel thickness in such small scale
structures. As a consequence of overlapped electrical double layer (EDL) in nanochannels
(typically < 50 nm), the ion perm-selective property creates an ion-flux imbalance between coand counter-ions at the inlet and outlet of the nanoscale junction. One of the major challenges
in homogeneous charged nanofluidic system is that the device must be operated under nonsteady state conditions (with cation selective junction)4, 13. Under such systems, the ion
depletion zone at the anodic side keeps ever-increasing, while the conductivity keeps
decreasing until the depletion zone becomes unstable.14 Especially, the amplified electric field
inside the depletion zone due to low electrical conductivity15 gave rise to an extremely fast
vortical motion and this convective fluid motion mainly drives the instability.16, 17 In addition,
ion enrichment at the cathodic side also leads to overall system instability in achieving efficient
ICP18 19. In order to overcome many of these limitations, heterogeneous nanoporous junctions
(with different polarity and different electroosmotic flow (EOF) direction) that are enable to
*Corresponding author: sky4u@snu.ac.kr: or jyhan@mit.edu.
†These authors equally contributed to this work.
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match up the excess ions allows stable ionic currents to be maintained through the junction
leading to stable operation. Despite its significant potential in various engineering applications,
however, there are only a few examples of heterogeneously charged nanoporous junction that
can exhibit ICP with either cation- or anion- perm-selectivity or both.
Hydrogels that are generated by polymeric cross-linking are attractive elements to nanoporous
junction due to their response to specific stimuli, such as pH or temperature changes.20-22 The
properties of hydrogels are determined by various parameters including monomer composition,
crosslinker density, and background ionic concentration.23 When the pore size of hydrogel
matrix becomes comparable to the charge screening length (Debye length) and the gel exhibits
many fixed charged groups, the hydrogel becomes a strong perm-selective material with
preferential counter-ion conductances24. Compared with lithographically defined
nanochannels25, 26 and non-crosslinked polymer materials (such as Nafion®)27 these
hydrogels can be structurally more robust, and cured into an arbitrary geometry and shape,
thereby providing greater flexibility. Most importantly, the polarity of the materials and the
surface charge density can be readily tuned, which is analogous to varying dopants (por ntypes) and doping density in semiconductors.
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Here, we investigated the effect of heterogeneously combined nanoporous junctions on the
stability of ICP. It is shown that positively and negatively charged nanoporous junctions, under
an applied electric field, are inducing ICP in the adjacent microchannel regions, with opposite
polarity. More specifically, positively charged junctions induce ion depletion (enrichment) in
the cathodic (anodic) side, as opposed to commonly used cation-selective nanoporous
junctions. An application of both junctions in a microfluidic system allows one to demonstrate
the effects of heterogeneous junction system on the stabilization of ICP, thereby introduceing
a new concept for ‘ion collector system’.
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To develop the ‘ion perm-selectivity’ attribute of the nanoporous junctions, we used positive
and/or negative charged hydrogels, HEMA-AA for cation selective hydrogel (CSH), and
DMAEMA for anion selective hydrogel (ASH) (see Methods and materials section). The ionic
hydrogels were embedded inside the PDMS substrate, followed by irreversible sealing with
the microchannel-patterned PDMS channel by plasma treatment, as shown in Figure 1a. These
ionic hydrogels have been shown to present a fixed charge inside matrix at optimal pH
conditions, owing to the deprotonation / protonation of acidic- / amine- groups28. The charge
state of the chemical groups determines cationic or anionic perm-selectivity of the nanopores.
The volume or nanopores size changes in this system, which is generated from swelling, would
not be a significant factor, because the hydrogel is tightly confined within the micropatterned
PDMS and strongly attached by oxygen plasma as shown in Figure 1b and 1c. Therefore, the
swelling should be minimized compared to the volume change in an open-environment
system29, and the fluid flows through microchannels without physical interruption (from the
swollen gel) as compared to previous approaches30, 31
In order to demonstrate a polarity of cationic selective hydrogel (CSH) junctions, we conducted
measurements of ionic current density through the hydrogel nanoporous junction (width: 50
μm, depth: 10 μm) which connected two microchannels with three different ionic strengths; 1
mM, 10 mM and 100 mM KCl solutions as shown in Figure 2a. The microchannels used in
CSH junction device had the dimension of 20μm height, 50μm width,, and 150μm spacing
between each microchannel. A decrease in the ion currents as a function of time through
nanoporous junctions under a dc bias is evidence of the formation of an ion depletion zone or
the initiation of concentration polarization14. At low buffer concentration (for 1 mM and 10
mM), the current quickly dropped to its steady value, whereas it slowly decreased in the case
of 100 mM because it takes longer to reach the proper thickness of EDL.
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One of the important characteristic properties of ionic flow through nanoscale junction (or
nanochannel) is that the ionic current versus applied voltage through the membrane has three
distinguishable regions called Ohmic / limiting / over-limiting current regions.32 At low
voltages, the IV relation typical follows Ohm’s law (Ohmic current region). At moderate
voltages, the current can not increase linearly with the voltage due to an ion depletion zone at
anodic side (limiting current region). In the over-limiting current region, the ionic current
through perm-selective membrane significantly increases because of convective mixing inside
ion depletion zone14, 32. We conducted another current measurement experiment for
investigating the current behavior at three different ionic strength, 1 mM, 10 mM and 100 mM
as shown in Figure 2b. Voltage, V, was swept from 0 V to 20 V at the rate of 0.1 V/30 sec
across the junction. Since the hydrogel structure has highly negative charges by acrylic acid
and the aforementioned effects, we can achieve three distinguishable regions even with 100
mM solution. Current studies have suggested that the model of the perm-selectivity of nanoscale structure originating from EDL overlap is not necessary the complete picture14,33, 34
Rather, highly charged surface of nano structure can sustain (non-electroneutral) surface ion
current, which can exist even when the equilibrium EDL is not ‘overlapping’ per se. This can
be proven by the observation that depletion / enrichment can be generated even at high ionic
strength (such as 100mM and even 1M), depending on the surface charge density of the
nanojunction. Compared to silicon or glass device, the hydrogel can inherently have highly
charged surface by adjusting the composition of charged additives. Thus, ICP can be initiated
even with 100 mM solution in this hydrogel network device.
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As a result, both the ion concentration and conductivity within the CSH decrease further,
leading to higher perm-selectivity junction. In such cases, the overall ion current could drop
to almost zero as shown in Figure 2a (their values were still non-zero because the applying
voltage of 50 V can initiate “over-limiting current behavior” which imposes much higher
current value than limiting current value), which can be viewed as analogous to a ‘reverse bias’
situation. In order to confirm the existence of both ion depletion/enrichment zones, ionic
currents through either anodic- or cathodic- side microchannel were measured after applying
an electric field through the nanoporous junction for a certain period of time (4 minutes for
Figure 2c and 8 minutes for Figure 2d). Here, 1 mM KCl buffer solution was used for the
measurement. This can be done by changing the voltage configuration from stage 1 (The
voltage is applied through junctions) to stage 2 (The voltage is applied between the
microchannel) (see the supplemental figure 2). As shown in Figure 2(a), the current reached
its steady value within 20 sec, and thus 4 and 8 minutes were enough for obtaining steady
current values. Moreover, this steady value largely depends on the (relatively high) resistance
of depletion zone, not the (relatively low) resistance of enrichment zone. Thus, the constant
current value before t = 0 (current value at the end of stage 1) can be confirmed by the initial
current value of anodic side (depletion side), 3.66 nA for 4 min, and 3.16 nA for 8 min, (which
are roughly the same) respectively. In the depletion zone, the initial low current increased up
to the reference value which was measured through a microchannel having uniform electrolyte
concentration. This is because the high resistance of depletion zone was getting lower as they
flew and removed toward the reservoir. In this sense, the depletion (enrichment) zone was
formed at the anodic (cathodic) side as shown in Figure 2c (Figure 2d) in the case of CSH
junction. Also, the maintaining stage 1 longer can give higher enrichment factors as shown in
Figure 2d as judged by higher initial current value at the cathodic side.
In the experiment, the cationic selective nanoporous junction (CSH, cationic selective
hydrogel) and anionic selective nanoporous junction (ASH, anionic selective hydrogel) show
opposite characteristics in the generation of ICP. Typically, surfaces charge distribution is one
of critical factors in EOF 36. In the case of CSH, the surface charge at CSH is negative, which
induces the ion depletion zone formed at the anodic side, similar to lithographically defined
nanochannel or Nafion nanojunction. The PDMS channel is also weakly negatively charged,
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which leads to an overall electroosmotic flow toward the cathode, pumping the desalted
(depleted) portion of fluid into the CSH junction. However, the situation with ASH junction
is more complex due to the different surface charge of ASH and PDMS itself. Namely, EOF
inside the ASH junction (unEOF=μnE) points toward the anodic side, while EOF inside
microchannel (umEOF=μmE) points toward the cathodic side. The EOF magnitudes are mainly
determined by spatial and temporal distribution of the surrounding electrolyte concentrations
as shown in Figure 3a. Here, the electroosmotic mobilities (μn and μm) are functions of the
zeta potential as well as E, i.e., the EOF velocity does not follow the Smoluchowski relation
inside the depletion zone and nanoscale junctions.15, 37 As shown in the inset of Figure 3b, the
three different current regions (Ohmic, limiting, and overlimiting) are also observed in the case
of ASH junction, and the ionic current through ASH junction initially dropped due to the
formation of a depletion zone in the cathodic side. Here, the electrolyte concentration of 1 mM
KCl solution was used. The reverse polarity of the ASH junction (depletion at cathodic side
and enrichment at anodic side) can also be confirmed by fluorescent image tracking as shown
in Figure 3c (at t=10sec). Ion enrichment at the anodic side decreases the electric field as well
as the zeta potential in the microchannel (anodic side), therefore reducing umEOF(EOF inside
the microchannel). In addition, due to ion depletion in the cathodic side, both the zeta potential
and the electric field increase, which increase unEOF (EOF inside the nanoporous juncrions)
The EOF generated within the ASH junction then becomes dominant (umEOF << unEOF), which
makes the overall EOF to point toward the anode. From this moment, the depletion zone at the
cathodic side is limited to a certain distance. As shown in the figure 3c at t=60 sec, the shape
of depletion boundary is a convex lens rather than parabolic usually observed in a CSH device.
This is because the overall flow field is subject to the competition between umEOF (pushing
toward cathodic reservoir) and unEOF (pulling toward nanoporous junction). The overall current
was measured increasing after 250 seconds as shown in Figure 3b. The ion-flux through each
cathodic- and anodic- microchannel was also monitored by ion current. In this sense,
enrichment behavior was observed at the anodic side and a depletion zone was created at the
cathodic side. After maintaining stage 1 for 4 minutes, the current through cathodic/anodic
microchannel followed typical depletion / enrichment situation, although with reversed polarity
(cathodic depletion and anodic enrichment) as shown in Figure 3d. However, the same
measurements at 8 minutes show much more complicated picture. The cathodic side
microchannel current was measured to go above the reference current value as shown in Figure
3e, suggesting a substantial enrichment of ions on the cathodic channel (outside the observed
depletion zone). Each reservoir at the end of anodic side can hold approximately 100 μL of
buffer solution, and the portion of the fluid volume, which can possibly flow at least 30 minutes,
was entirely enriched during the first 8 minutes of maintaining stage 1. Thus the current went
up to the reference value (Figure 3e), while the current level with 4 minutes of stage 1 (Figure
3d) would not go above the reference value. Overall, the ion currents across the system were
maintained near or above the steady state level, which is characteristically different from the
CSH junction results. The opposite polarity of two different nanoporous junctions can lead to
EOF with different direction in the device, which introduce a stable ion – flux within the device.
In order to test the opposite polarity of two difference nanojunction, we designed two different
types of nanojunction devices: one is with junctions of the same charge (CSH-CSH junction)
and the other with the oppositely charged junctions (CSH-ASH junction). The geometrical
configuration of each device was identical (100 μm distance between each junction). Figure
4a shows the ionic current through the CSH-ASH hybrid junctions system with 1 mM KCl
buffer solution. Initially, both the CSH and the ASH junctions operated and created ion ICP
zones according to their own polarity, resulting in anodic depletion and cathodic enrichment
as shown Figure 4b. However, as shown in Figure 4a, the overall ion current quickly returns
to a steady state value that is only slightly lower than the ion current of uniform concentration.
Therefore, the usual precipitous conductance drop, which is typically observed in devices using
a single perm-selective nanochannels and nanopores, is not observed and a high level of
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conductance is maintained. To investigate further details of ionic distribution both on the anodic
and cathodic side, the currents through each side at (i) (4 minutes) and (ii) (18 minutes) were
measured, as shown in Supplemental Figure 3a and 3b, respectively. The overall concentration
profile in the cathodic (enrichment) and the anodic (depletion) side of the nanoporous junction
is maintained. This is because ion enrichment (from the CSH junction) is mitigated by the
action of the opposite ASH junction, which pumps extra anions to the opposite side. In order
to verify the stabilization of ICP in the CSH-ASH hybrid nanoporous junction, green
fluorescence protein (GFP) was introduced at both anodic and cathodic side. Oppositely
charged ion tunnels can hinder or eliminate individual ionic transportation as shown in Figure
4c. Overall, the CSH junction is more conductive than the ASH junction, leading to cathodic
enrichment and anodic depletion. This is because CSH junction (pKa=4.3 @ 25 °C) is more
conductive than ASH junction (pKa=8.4) at this pH=7.4 condition. The CSH was
approximately 2 times more conductive than ASH, as shown in the I-V slopes in the Ohmic
regime of each nanoporous – junctions (Figure 2b for CSH and inset of Figure 3b for ASH).
However, continuous concentration polarization is prevented by the current through ASH
junction, mitigating any further ion concentration changes and maintains the size of the
depletion / enrichment zones. As a result, ion current in this system reaches a steady state,
which can be represented by the steady accumulation of molecules as a function of time. Such
a condition was not obtained when two identical CSH junctions were used as shown in Figure
4d. This result clearly demonstrates that the advanced control of ion – flux can be obtained by
integrating the opposite ion transport properties of CSH and ASH junctions. Also we performed
experiments with various distances (50 μm, 100 μm, 500 μm, and 1 mm) between ASH and
CSH. The results demonstrated that there was no interaction for a larger distance than 100
μm. It appeared that each junction acted independently and did not interact with each other
based on the observation of the florescent images (See Supplemental Movie 3 where the gap
between ASH and CSH is 1mm).
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Some comments follow with regard to the advantageous aspect of hydrogels as an alternative
to nanoporous junction device. It is worthwhile noting that the relative strength of the two
opposite junctions can be tuned as needed, by changing the monomer composition of the
hydrogel, or by increasing/decreasing the number of junctions. Such a manipulation would
provide a new mode of ion – flux control, potentially leading to new device concepts. For
example, one could build an ASH-CSH-ASH hybrid junction system, as shown in Figure 5.
This system is ‘designed’ to have the dominant ion transport properties of ASH junctions, but
with a CSH junction mitigating and maintaining the stability of the system (Figure 5a).
Characterization of ion concentration distributions and overall ion transport behaviors are
shown in Figure 5b~e. Here, the electrolyte concentration of 1 mM KCl solution was used.
The unique feature of this system, as seen in Figure 5b, is that overall ion current through this
system actually increases as a function of time, above the level predicted by uniform
distribution of ions across the entire system. Also the weak ion depletion zone was initially
formed at the cathodic side, while the strong enrichment zone was initially formed at the anodic
side. This is similar behavior to the single ASH type junction because most of current pass
through the shortest path, (along the left edge of the ASH hydrogel, which would form a leastresistance path between the left anodic reservoir and the left cathodic reservoir involved and
vice versa) i.e. no current flow through center CSH. However, the weak depletion zone on the
cathodic side (which was probably the limiting factor for the case of ASH-only junctionsFigure 3) was gradually eliminated and turned into an almost- uniform enrichment zone. The
current through CSH junction in the middle may work to limit the extent of cathodic depletion
zone. The overall conductance of the system continuously increased (i.e., working as an ‘ion
collector’), since both side of the junction work as enrichment zones. This can be confirmed
by current measurements in the microchannel, as shown in Figure 5c~5e. During the first 8~10
minutes, one can see that the initial polarization (both cathodic depletion and anodic
enrichment) is slowly decreasing, while the concentration polarization is still local (shown by
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the same steady state current of 30nA). However, by 18 minutes, it was shown that the weak
depletion zone at the cathodic side (Figure 5c) was changed into the enrichment zone (Figure
5e). These effects gave rise to a higher ionic current through the nanoporous junctions without
instability of ICP.
In summary, we have presented negative- and/or positive- charged nanoporous junctions inside
microfluidics channel network by simple soft-lithographic integration and characterized the
ICP phenomena in these complex systems. Utilizing this nanoporous junction system, we first
demonstrated the reversed polarity of ICP in an anionic selective junction. This reversed
polarity played a role in stabilizing the ICP by pumping anions to the opposite side, thus
allowing for the generation of a controlled ion-pathway in an electrofluidic system. Such
characterizations of hybrid nanoporous junction system (i.e., CSH-ASH and ASH-CSH-ASH)
promise a great potential for manipulation of ions and biomolecules in various membrane
applications, such in battery and desalination membranes.
Methods and materials section
Materials
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Polydimethyl siloxane (PDMS) prepolymer and curing agent were purchased from Dow
Corning (Sylgard 184). GFP for fluorescent tracking was purchased from CloneTech. For
cationic selective hydrogels, a precursor mixture was used, consisting of 2-hydroxyethyl
methacrylate (HEMA) and acrylic acid (AA) in the weight ratio of 5:1. The materials were
purchased from Sigma-Aldrich Co. and vacuum-distilled prior to use. In the solution, a
crosslinker (1 wt% ethylene glycol dimethacrylate (EGDMA)) and a photoinitiator (3 wt%
2,2-dimethoxy-2-phenyl-acetophenone(DMPA)) were used as the materials for photopolymerizing (Signal-Aldrich Co.). The anionic selective hydrogel pre-polymer mixture
consists of HEMA, 2-(dimethylamino)ethyl methacrylate (DMAEMA), EGDMA and DMPA
in the weight ratios of 30.2:6.16:0.5:1.0, respectively.
Device Fabrication
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To cure the PDMS prepolymer, a mixture of 10:1 silicone elastomer and curing agent was
poured on various silicon masters prepared by photolithography and dry etching and placed at
60°C for 2 hr. The masters used for microfluidic channel had two branch channels with the
dimension of 50 ~ 100 μm in width and 20 μm in height. The two channels were 100 μm apart.
The maters used for hydrogel junctions had 50 μm in width, 10 μm in depth, and 100 μm in
spacing between the adjacent junctions. Single or multiple channels were used depending on
the construction geometry of the junction device. After curing, PDMS molds were cleanly
detached from the masters. Glass slides were prepared by washing in distilled water and cleaned
by plasma for 1 min. Initially, the acid and base monomers containing photo-curable hydrogels
were pre-patterned on PDMS substrate, using the method called “micromolding in capillaries
(MIMIC)”38 The defined structure of hydrogel on PDMS surface was formed by rapid
polymerization under ultraviolet (UV) light. Prior to introducing the hydrogel mixture, the prepatterned PDMS channel was treated by controlled oxygen plasma, which allowed for strong
adhesion between the pre-patterned PDMS surface and the cured hydrogel structure after
polymerization. The conditions used were tuv, HEMA-DMAEMA = 60 s, and tuv, HEMA-AA = 40
s at a constant UV intensity of 10 mW/cm2. The microfluidic PDMS channel and the bottom
ionic-hydrogel embedded PDMS substrate were irreversibly bonded with O2 plasma treatment
for 60 s (PDC-32G, Harrick Scientific, Ossining, NY).
Electrokinetic Process
The ion depletion/enrichment zones were visualized by tracking fluorescence dye molecules
and proteins that were added in the main 1 mM KCl solution at pH = 7.5. All the flow motions
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and ion transport were imaged with an inverted fluorescence microscope (Olympus, IX-51)
with CCD camera (SensiCam, Cooke corp.). Sequences of images were analyzed by Image
Pro Plus 5.0 (Media Cybernetics inc.). A dc power supplier (Stanford Research System, Inc.)
was used to apply an electrical potential to each reservoir through a homemade voltage divider.
Keithley 236 Current/Voltage Source-Measure Unit (Keithley Instruments, Inc.) was used to
measure the ionic current either through the hydrogel or microchannels
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea
government (MOST) (R01-2007-000-20675-0) and the Korean Research Foundation Grants funded by the Korean
Government (MOEHRD) (KRF-2007-331-D00040 and KRF-J03000). This work was also partially supported by NIH
(EB005743) and NSF (CBET-0347348).
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Figure 1.
Fabrication of a nanoporous junction hybrid device with ionic hydrogels. (a) optical
micrograph showing the top view of the hybrid device, and (b) its cross section at the location
indicated by A-A’. Cationic selective hydrogel (CSH, fixed group: −COO−) and anionic
selective hydrogel (ASH, fixed group: −NH3+) were used for the hybrid device.
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Figure 2.
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Ionic current measurement of the CSH junction. (a) Ion current density through nanoporous –
junction for various ion concentration (buffer concentration) (b) Current sweep plot showing
the limiting current and overlimiting current pattern for three different concentrations. The
voltage sweep was done by a programmed voltage source (current-source measurement unit
236, Keithley Instruments, Inc., OH). The voltage was ramped from 0 to 20 V, 0.1 V / 30sec.
Plot of ion current through each microchannel for investigating the formation of ion-depletion /
enrichment zone after maintaining voltage through nano-junction for 4 minutes (c) and 8
minutes (d) (V: voltage, G: electrical ground and F: electrical floating). 1 mM KCl buffer
solution was used for (c) and (d).
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Figure 3.
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Ionic current measurement and fluorescent tracking of the ASH type junction. (a) Schematic
illustration of EOF through nanoporous junctions. (b) Ion current through ASH junction as a
function of time. (c) Fluorescent image tracking of ion depletion / ion enrichment zone in ASH
system as a function of time. (d) and (e) Plot of ion current through each microchannel to
investigate the formation of ion- depletion / enrichment zones after maintaining voltage through
the nanoporous junction for 4 minutes (d) and 8 minutes (e). (Also, see Supplemental Movie
1) 1 mM KCl buffer solution was used.
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Figure 4.
Ion movement of the ASH-CSH hybrid junction. (a) Ion current through the ASH-CSH junction
as a function of time. (b) Schematic illustration of ion transport through the ASH-CSH hybrid
system. Experimental observation of ion transport through ASH-CSH hybrid system (c) and
CSH-CSH (d). The ion enrichment zone at the cathodic side of CSH junction disappears due
to strong ion depletion from the ASH junction. (Also, see Supplemental Movie 2 and Movie
4) 1 mM KCl buffer solution was used.
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Figure 5.
Ionic current measurement of the ASH-CSH-ASH hybrid junction. (a) Schematic illustration
of ion transport through ASH-CSH hybrid system (b) Ion current through the ASH-CSH-ASH
hybrid junction as a function of time. (c-e), Plot of ion current through each microchannel to
investigate the formation of ion- depletion / enrichment zones after maintaining voltage through
the nanoporous junction for 4 minutes (c), 8 minutes (d) and 18 minutes (e). 1 mM KCl buffer
solution was used.
Nano Lett. Author manuscript; available in PMC 2011 January 1.